The aim of this proposal is to study the developmental regulation of dendritic spine plasticity and the role of experience in modifying synaptic connections in postnatal life. Using an in vivo transcranial two-photon imaging technique, changes of individual dendritic spines will be followed over extended periods of time in different cell types and in diverse cortical regions. We will determine how and to what degree sensory experience and learning modulate dendritic spine plasticity in the developing and adult cortex. Furthermore, we will investigate whether experience-dependent spine plasticity is compromised in mouse models of Fragile X syndrome and Rett syndrome and if so, whether drug treatment can restore experience-dependent spine plasticity in mutant mice. Together, these studies will provide fundamental insights into how experience and genetic factors shape neural circuits at different stages of life and suggest new strategies for the treatment of mental retardation.